Air entrainment is important for increasing resistance of hydraulic cementitious compositions, such as mortar, masonry, and concrete, to frost attack and deterioration due to repeated freezing and thawing. Entrained air is desirable, therefore, for long-term durability of concrete or mortar in adverse freeze-thaw conditions.
Technically speaking, air entraining agents or admixtures (AEAs) do not generate air in the concrete, but merely stabilize the air either (1) infolded and mechanically enveloped during mixing, (2) dissolved in the mix water, (3) originally present in the intergranular spaces in the dry cement and aggregate; or (4) in the pores of the aggregate. While it is true that the entrained air is within the total mass of concrete, it is only entrained in the paste portion of the mix.
Entrained air is fundamentally different than "entrapped" air. Most of the air that is in concretes, in the absence of admixtures, is often referred to as "entrapped" air. Entrained air is characterized by uniformly dispersed, spherical spaces in cement paste; whereas entrapped air is characterized by irregularly shaped voids which are not generally uniformly sized but generally larger than "entrained" air voids. See e.g., Concrete Admixtures, Vance Dodson (Van Nostrand Reinhold, New York 1990), P. 129 et seq.
The present inventors have discovered an air entrainment problem that arises when oxyalkylene based shrinkage reducing admixtures (SRAs), in concrete and mortar, are used in conjunction with water soluble salts. Oxyalkylene SRAs are known in the art. For example, U.S. Pat. No. 5,413,634 of Shawl et al., incorporated herein by reference, discloses an alkyl ether derivative of an aliphatic polyhydroxy compound containing an oxyalkylene group.
When an oxyalkylene SRA is used with a water soluble salt (such as calcium nitrite), however, sufficient and controllable levels of entrained air are difficult to attain. This was found to be the case even when the SRAs were used in combination with conventional air entraining agents, such as tall oil and vinsol resin. Accordingly, a novel air entraining admixture or system is needed.